Based on scary specimens spotted in refrigerators and deserted lunch bags, you may think of mold as soggy, dirty, disgusting and, above all, obvious. However, that element of the obvious isn’t always present in some household mold mysteries. The most immaculate, well-kept home can have serious mold issues -- infestations that threaten human health and impact the systems and structure of the house itself.

Take my friend Maria Sherow and her previous home in Harding Township, New Jersey. Her son’s asthma symptoms had worsened since the family’s move to the house, and after no luck locating any mold, she asked me to put on my professional home inspector cap and take a closer look. Near the end of a fairly uneventful inspection, I happened to grab a handful of the blown-in insulation lining Maria’s attic, and sent it off to air quality consultant and mold expert Jeff May for testing. Not long after that sample landed in the lab, I got a call from Jeff, who told me that the insulation was loaded with mold.

How did it get there? Turn’s out that fiberglass insulation, which by itself won’t grow mold, is a pretty good filter material. Since this house had many recessed lights, dust escaped through those lights into the insulation, where it served as mold-food. Making maters worse, the attic space did not have enough ventilation, which kept the insulation damp. Bottom line: dust plus moisture equaled enough mold to completely infest the well-kept house and require a major cleanup, even though the problem was never obvious.

“If you looked at the home, you would never, ever in a million years guess that it had a mold problem,” says Maria. “It didn’t look moldy -- you couldn’t see the traditional mold because it was actually hidden inside the insulation fibers. It just goes to show that you don’t necessarily see mold. It can be an invisible threat just like carbon monoxide.”

In addition to this attic example, there are several other hidden hideouts for mold in the typical home. All mold needs to grow is moisture, air and food -- that is, biodegradable substances like paper, fibers and dust. Here are some unusual suspects in the investigation of mold issues:

Basements: Whether they’re finished of unfinished, basements should be subject to climate control that can prevent mold development. “Unfinished basements must be adequately dehumidified in the humid season (relative humidity kept under 50 percent) to prevent mold growth on biodegradable dust,” advises Jeff. “That includes house dust and sawdust captured in exposed fiberglass insulation and stuck to the concrete walls and floor.”

Also be careful about what you store in a basement and how you store it, as cardboard boxes and other fiber-based items provide the kind of food that’s desirable to mold.

Kitchens: Improper or inadequate ventilation can put mold in contact with your food supply and preparation, no how matter how clean you keep your kitchen. And don’t leave those leftovers too long in the refrigerator, as the mold that may develop can sneak into nooks and crannies of the appliance and lead to ongoing food safety issues.

In-room humidifiers: While humidifiers can be a help for some health issues, they can create completely new ones if not used and maintained properly. Elevating the relative humidity in a room can be an invitation for the sneakiest forms of mold, and a humidifier unit that goes a long time between cleanings can harbor and broadcast mold.

Leaky chimneys: An out-of-repair chimney is a threat not only because of the carbon monoxide it can release into a home, but also for the mold it can hold. Moisture that collects in internal nooks and crannies of a leaky chimney can lead to mold colonies that spread into nearby walls and ventilation outlets.

Lighting fixtures: Strange as it may sound, even lighting fixtures can hold onto mold. As the Sherow family found out, mold develops in the insulation surrounding recessed lights, finding its way out to living areas through insufficiently sealed fixture housings.

Air conditioning equipment: What you don’t see can definitely hurt you when it comes to air conditioning equipment, with mold gathering on coils and in other areas that come in contact with moisture. Establish a careful maintenance routine during the AC season to ensure that cool air doesn’t carry mold along with it.

If, like Maria, you sense the presence of mold in your home but can’t track it down, work with a professional to find out where mold has taken hold. A home inspector who is trained in mold detection and a member of the American Society of Home Inspectors (ASHI) can help you find the problem and change the conditions so that the mold doesn’t return. Otherwise, stay vigilant in guarding your home against mold development by minimizing its mold-friendliness.

“It doesn’t matter how clean your home is,” says Maria. “If you have a water leak of any kind, that’s enough for mold to grow -- enough for you to have toxic conditions and all kinds of health problems. So you just have to really look around and ask yourself, ‘Do I have any of these conditions in my home?’ and if so, you have to dry them out, taking the food away from the mold so the mold can’t grow.”

Mold could be hidden out of sight in your otherwise clean home. For the health and safety of your family, find out where the mold is hiding in your house.